Digital storage oscilloscopes (DSO) are used to test electronic devices for operational performance. A device under test is operated, and probes connected to the DSO display the signal at selected outputs or nodes on the device. The signal is displayed on a display screen of the DSO for analysis. A typical DSO display has a time domain on the horizontal axis, and voltage domain on the vertical. For high-frequency digital logic signal analysis, the time axis is established to relate to a device clock signal, such that repetitious signal activity such as switching between high and low logic states are overlaid in alignment with prior signals, so that each possible logic state transition is aligned with the possible transitions of prior clock cycles.
For the above example, a mask is provided to indicate for each point along the time domain prior to, during, and after a switching event, what voltages are acceptable, and what are not. For instance, before and after the time of switching, the accepted voltages may be within a tolerated range of either the nominal logic low voltage and the nominal logic high voltage, but in violation of the mask if below the lower limit of the low range, between the two ranges, or above the upper limit of the upper range. For a limited time before and after the expected switching time, a voltage between the ranges is also acceptable. In typical such masks, there is a temporal transition that tolerates limited deviations from the low and high ranges just before and after the transition zone, so that normally sloped (non-square, non-instantaneous) transitions are tolerated by the mask.
Often, a violation by a signal of the mask is not large and conspicuous. For instance, a “ringing” in a signal that leads the signal to momentarily overshoot the desired voltage after a transition may violate the mask by a very small amount that is difficult to see on the display screen. The violation may be only one pixel, or another small number of pixels, on a high-resolution screen having very small pixels. The mask is normally displayed, as are grid lines and other indicia, which may conceal a slight mask violation.
Existing systems highlight violations by the use of a color for the violative pixels that differs from the normal color of the compliant signal trace. While effective for large, conspicuous violations, this is ineffective for small or slight violations that are difficult for a user to visualize. Moreover, these difficulties are worse when the DSO is displaying thousands of overlaid traces on the same mask, so that individual traces may not be discerned. In addition, where there are several violations, large conspicuous ones may distract from or obliterate smaller ones of importance.